Heretofore, some sealed batteries are provided with safety valves on outer peripheral surfaces of the batteries. The safety valve opens when pressure inside the battery rises and exceeds a tolerable level. The safety valve thus serves as an opening for discharging gas and others existing inside the battery. Accordingly, a valve opening pressure is desired to be at a target level.
There has been proposed in Patent Literatures 1 and 2 that a safety valve has, for example, a thin-wall portion and the thin-wall portion is internally formed with a thin grooved portion. For obtaining such a safety valve with stable product quality, it is desirable to make the groove width narrow. This is because larger groove width could result in unevenness of the valve opening pressure.